Coated tools are generally known in which a hard coating layer composed of the following (a) and (b) is deposited on a surface of a body made of a tungsten carbide (hereinafter, expressed by WC)-based cemented carbide or a titanium carbonitride (hereinafter, expressed by TiCN)-based cermet (hereinafter, these will be collectively referred to as a tool body).
(a) A Ti compound layer as a lower layer composed of one or more of a Ti carbide (hereinafter, expressed by TiC) layer, a Ti nitride (hereinafter, similarly expressed by TiN) layer, a Ti carbonitride (hereinafter, expressed by TiCN) layer, a Ti oxycarbide (hereinafter, expressed by TiCO) layer, and a Ti oxycarbonitride (hereinafter, expressed by TiCNO) layer.
(b) An aluminum oxide layer (hereinafter, expressed by an Al2O3 layer) as an upper layer having an α-type crystal structure in a chemically deposited state.
However, the above-described conventional coated tools exhibit excellent wear resistance in, for example, continuous cutting of various steels, cast irons, and the like, but in a case where the coated tool is used in high-speed intermittent cutting, peeling or chipping of the coating layer easily occurs, and there is a problem in that the tool life is reduced.
Therefore, various coated tools having improved upper and lower layers have been proposed in order to suppress peeling and chipping of the coating layer.
For example, Japanese Unexamined Patent Application, First Publication No. 2006-198735 discloses a coated tool obtained by depositing a hard coating layer composed of the following (a) and (b) on a surface of a tool body made of a WC-based cemented carbide or a TiCN-based cermet.
(a) A Ti compound layer as a lower layer composed of one or more of a Ti carbide layer, a Ti nitride layer, a Ti carbonitride layer, a Ti oxycarbide layer, and a Ti oxycarbonitride layer and having an average total layer thickness of 3 to 20 μm.
(b) An aluminum oxide layer as an upper layer with an average layer thickness of 1 to 15 μm having an α-type crystal structure in a chemically deposited state. Regarding this upper layer, a highest peak is present in Σ3 and a distribution ratio of Σ3 in the whole of ΣN+1 is 60 to 80% in a constituent atom-sharing lattice point distribution graph showing distribution ratios of individuals of ΣN+1 to the whole of ΣN+1, when electron beams are irradiated to the individual crystal grains having a corundum hexagonal crystal lattice in a measurement range of a polished surface by using a field-emission-type scanning electron microscope to measure inclined angles between normal lines of the (0001) plane and the (10-10) plane, which are crystal planes of the crystal grains, and a normal line of the polished surface; the crystal grains of this case have a corundum hexagonal crystal structure in which constituent atoms composed of Al and oxygen are present at lattice points, respectively; the distribution of coincidence grain boundary formed of lattice points (constituent atom-sharing lattice points) where the respective constituent atoms share one constituent atom between the crystal grains at an interface between the adjacent crystal grains is calculated on the basis of the resulting measured inclined angles; and when ΣN+1 represents the coincidence grain boundary formed of a constituent atom-sharing lattice point type in which there are N lattice points sharing no constituent atoms between the constituent atom-sharing lattice points (here, N is any even number equal to or more than 2 in the corundum hexagonal close-packed crystal structure, but in a case where the upper limit of N is 28 from the viewpoint of distribution frequency, even numbers 4, 8, 14, 24, and 26 do not exist).
This coated tool is known to exhibit excellent chipping resistance in high-speed intermittent cutting work.
PCT International Publication No. WO2013/038000 proposes that in a coated tool in which a surface of a tool body is coated with a lower layer and an aluminum oxide layer, or in a coated tool in which an interlayer interposed between a tool body and the lower layer is coated with an aluminum oxide layer formed on a lower layer, chipping resistance and crater wear resistance are improved by setting a Σ3-coincidence grain boundary ratio of the aluminum oxide layer to 80% or greater.
Japanese Unexamined Patent Application, First Publication No. 2013-63504 discloses a surface-coated cutting tool in which a hard coating layer composed of a Ti compound layer as a lower layer and an α-type Al2O3 layer as an upper layer is deposited, in which 30 to 70 area % of Al2O3 crystal grains immediately above the lower layer is constituted of (11-20)-oriented Al2O3 crystal grains, at least 45 area % of all Al2O3 crystal grains of the upper layer is constituted of (0001)-oriented Al2O3 crystal grains, and more preferably, an outermost surface layer of the lower layer is constituted of an oxygen-containing TiCN layer containing 0.5 to 3 atom % of oxygen only in a depth region with a depth of up to 500 nm. A value of a ratio between the number of oxygen-containing TiCN crystal grains of the outermost surface layer of the lower layer and the number of Al2O3 crystal grains at an interface between the lower layer and the upper layer is 0.01 to 0.5. Accordingly, it is proposed to improve peeling resistance and chipping resistance of the surface-coated cutting tool in high-speed heavy cutting and high-speed intermittent cutting.